The invention relates to the formulation of a starting mixture for a dielectric composition, comprising a vitreous phase and several ceramic phases in the respective ratios by volume for the vitreous phase between 85 and 60% and for the ceramic phases between 15 and 40%, the vitreous phase being a silicate of zinc and alkaline earth.
The invention is useful in the manufacture of power microcircuits or a support of a large surface area for components in the field of integrated circuits.
It is known from the article by J. P. DELLA MUSSIA in "Electronique actualites" of Mar. 9, 1984, p. 15, entitled "Les telecommunications etrangeres homologuent les co-lamines d'Imphy" to substitute on the conventional ceramic substrate which constitutes the support of the assembly of the integrated circuits a new conductive laminate, covered by an electrically insulating layer.
This laminate is constituted by a substrate of an alloy of the Invar type (nickel-iron alloy containing 36% of nickel), each of the surfaces of which is provided with a copper plating thus forming a succession of three layers, copper, Invar, copper. The laminate has a coefficient of expansion which is adjustable during its manufacture due to the fact that this coefficient depends on the relative thickness of the three layers.
A laminate in which the triplet Cu/Invar/Cu has the thickness percentages 16/68/16, has a number of advantages. Among these advantages are:
a coefficient of expansion, parallel to the surface, very near that of alumina and various semiconductors, between 0.degree. and 300.degree. C., PA0 a thermal conductivity, parallel to the surface, approximately seven times greater than that of alumina (while the thermal conductivity perpendicularly to the surface is of the same order of magnitude as that of alumina). An essential function of this laminate thus is to serve as a radiator for the circuits which are disposed thereon; PA0 the possibility of providing substrates of large dimensions, which is not the case with alumina; PA0 the possibility of providing substrates as thin as the substrates of alumina, for example 35 .mu.m; PA0 the possibility of soldering on the rear side a supplementary layer of copper in order to improve the radiation properties; PA0 resistance to shocks and PA0 the possibility of mechanical or chemical piercing, folding, stamping or cutting. PA0 the poor temperature behaviour of such materials, the polymerization temperature being of the order of 250.degree. C.; PA0 the poor mechanical rigidity of the polymerisable materials, which does not permit taking advantage of the great rigidity of the substrate; PA0 the bad electrical conductivity of the conductive layers which, being polymers charged with silver, shows the following surface resistivity: PA0 R.perspectiveto.30 m .OMEGA./square; PA0 the fact that these conductors cannot be soldered and PA0 the lack of fineness of the resultant conductors, which does not permit circuits of high integration density to be devised. PA0 a high firing temperature thereby being able to withstand the refiring of the envisaged conductive layer(s); PA0 a coefficient of expansion near that of the laminate not only in the range of temperature between 0.degree. C. and 300.degree. C. but also for temperatures between 300.degree. C. and the firing temperature of the ink; PA0 a good adherence to the laminate; PA0 and finally the possibility of being fired under a non-oxidising atmosphere so as to avoid oxidation of the laminate.
Another laminate of the triplet Cu/Invar/Cu 16/68/16, in which the inner layer has a corrugated form, possesses enhanced refrigerating properties due to internal circulation. It presents, due to this fact, an extra advantage, and may serve as a substrate for circuits intended, for example, for military applications, having to satisfy the standards -55.degree. C.-+125.degree. C.
However, these laminates have the following disadvantages:
They are not electrically insulating at the surface. PA1 Oxides are formed at the surfaces if they are not protected. PA1 the possibility of withstanding high temperatures; PA1 a high mechanical rigidity; PA1 a fineness of the resultant conductors which permits circuits of high integration density to be obtained and PA1 a very good electrical conductivity of said layer of which the surface resistivity is: PA1 R.perspectiveto.2 m .OMEGA./square.
From these advantages and disadvantages if follows that the laminates have been provided originally to serve as a support and radiator for ceramic substrates, the so called "chip carrier" of integrated circuits, or for power components or components of large dimensions, due to the compatibility of the coefficients of expansion of the laminate and of these elements by forming at the surface of the laminate an electrically insulating layer on which the circuit of these elements is formed.
It is known, as described in the above-mentioned article to manufacture the insulating layer and the circuit by means of polymerisable organic materials.
However, this process has several disadvantages among which are:
On the contrary a conductive silk-screening ink such as that described in GB-PS 1,489,031 presents several advantages which include:
However such a conductive silk-screening ink cannot be used directly on the laminated substrate. An insulating layer which is also perfectly adapted to the laminate and to the conductive ink is preferably provided between the laminated substrate and the conductive silk screening ink.
Thus, the following technical problem is present: In order to profit by the advantages presented by the new laminated substrate while forming thereon circuits by means of a conductive silk-screening ink, as has been described, it is desirable to provide an insulating silk-screening ink (.epsilon. as low as possible) and which has:
A starting mixture for an insulating composition for use in an insulating silk-screening ink of high firing temperature under nitrogen is known from U.S. Pat. No. 4,323,652. According to this patent this mixture comprises a vitreous phase constituted by the molar ratios of the following oxides: from 30 to 55% of silica (SiO.sub.2), from 20 to 40% of zinc oxide (ZnO), from 0 to 20% of boric anhydride (B.sub.2 O.sub.3), from 0 to 10% of alumina, from 5 to 40% of calcium anhydride (CaO), strontium oxide (SrO) and barium oxide (BaO), as well as optionally from 0 to 10% of cobalt oxide (CoO) as a dye. This mixture also comprises several ceramic phases such as zinc oxide and cobalt oxide.
According to this patent by use of such a mixture in the ratios by volume between 85 and 60% for the vitreous phase and between 15 and 40% for the ceramic phases dispersed in an organic carrier, an insulating silk-screening ink may be obtained which is capable of being fired in a high temperature in nitrogen and compatible with the conductive ink as described. However, this ink is suitable only for the manufacture of hybrid microcircuits on ceramic substrates and always leads to a deformation of the substrate when it is used on the laminate.
In effect, in order to adapt said ink to the laminate substrate, tests of all possible variations of the ratios of the constituents of the ink suggested by the above-cited patent have been made. None of these ratios have proved to be successful.
For, of all the difficulties encountered by developing such an ink, the most complicated one to solve is the adaptation of the temperature coefficient of the ink to that of the laminate. At any rate the solution to the technical problem described is not found in the use of an ink manufactured according to the above-mentioned patent.